This invention relates generally to the field of chemical mechanical polishing (CMP) of semiconductor wafers or substrates. More specifically, the invention relates to a method of chemical mechanical polishing of semiconductor wafers or substrates.
The ever-increasing demand for high-performance microelectronic devices has motivated the semiconductor industry to design and manufacture Ultra-Large-Scale Integrated (ULSI) circuits with smaller feature size, higher resolution, denser packing, and multi-layer interconnects. The ULSI technology places stringent demands on global planarity on multiple layers, called Interlevel Dielectric (ILD) layers, which comprise the circuit. Compared with other planarization techniques, the chemical mechanical polishing (CMP) process produces excellent local and global planarization at low cost, and is thus widely adopted in many back-end processes for planarizing inter-level dielectric layers, which are most often silicon dioxide (SiO2). In addition to achieving global planarization, CMP is also critical to many emerging process technologies, such as the polishing of copper (Cu) damascene patterns, low-k dielectrics, and shallow trench isolation (STI) structures (Landis et al., 1992; Peters, 1998). The wide range of materials to be polished concurrently or sequentially, however, increases the complexity of the CMP process and necessitates an understanding of the process fundamentals for optimal process design and control.
Despite its extensive use in ULSI manufacturing, the basic material removal mechanisms in CMP are not yet well understood. Long ago, Preston empirically found in glass polishing that the material removal rate (MRR) is proportional to the product of the applied pressure and the relative velocity (Preston, 1927). The Preston equation may be written as             ⅆ      ξ              ⅆ      t        =            k      p        ⁢    p    ⁢          xe2x80x83        ⁢          v      R      
where "xgr" is the thickness of the layer removed, t the polishing time, p the nominal pressure, vR the relative velocity, and kp is a constant known as the Preston constant.
In recent years, it has been demonstrated in many works that the above relation is also valid for metals (Steigerwald et al., 1994; Stavreva et al., 1995 and 1997) and ceramics (Nakamura et al., 1985; Komanduri et al., 1996). To explain this proportionality, researches have attempted to study the material removal mechanisms during the CMP process, and several researchers have proposed particle abrasion (Brown et al., 1981; Liu et al., 1996) and pad asperity contact models (Yu et al., 1993) to elucidate the mechanical aspects of the CMP process. Assuming that wafer/abrasive or wafer/pad is in contact, the applied stress field near the wafer surface results in elastic-plastic deformation of the surface layer and produces wear. Another line of research has focused on the chemical mechanisms of the process (Cook, 1990; Luo et al., 1998). Cook first reviewed the chemical process for glass polishing. He suggested that both surface dissolution under particle impact and the absorption or dissolution of wear particles onto the slurry particles will determine the polishing rate of glass. More recently, a two-dimensional wafer-scale model based on lubrication theory (Runnels and Eyman, 1994) and mass transport has been proposed (Sundararajan et al., 1999). In this model, the wafer is assumed to hydroplane on the pad surface, and the normal load is supported by the hydrodynamic pressure of the viscous slurry film. The polishing rate is determined by the convective mass transport of the chemical species.
Whether material removal is by mechanical, chemical, or chemomechanical interactions in the CMP process, an understanding of the contact condition at the wafer/pad interface is important to process characterization, modeling, and optimization. However, to date there is no explicit methodology in the CMP literature to characterize wafer-scale interfacial conditions with process parameters. Some researchers have assumed that the wafer hydroplanes while being polished, and thus solve the Reynolds equation of lubrication to determine the relations among wafer curvature, applied pressure, relative velocity, slurry viscosity, slurry film thickness, and pressure distribution on the wafer surface (Runnel, 1994; Runnel and Eyman, 1994). Another group has assumed the wafer is in contact, or partially in contact with the pad, and relate the displacement of the wafer to the pad elastic modulus and solve the stress field by the classical contact mechanics model (Chekina et al., 1998). Measurement of the vertical displacement of the wafer relative to the pad seems the most direct prior art technique of identifying the contact condition and determine the slurry film thickness (Mess et al., 1997). However, the compliance of the pad material and that of the back film in the wafer carrier make such measurements unreliable. While some experiments in the hydroplaning mode have been conducted on smaller specimens (Nakamura et al., 1985), it is questionable to scale up the results to a larger size wafer. In general, different applied pressure, velocity, and other experimental conditions employed by the various investigators have resulted in a difficult situation to draw any definitive conclusions regarding the mode of interfacial contact. Thus, it is highly desirable to determine and characterize the primary material removal mechanism during CMW and to provide a CMU process that promote an increased material removal rate (MRR) from the surface of the wafer.
References discussing CMP processes in the semiconductor industry include:
Bhushan, M., Rouse, R., and Lukens, J. E., 1995, xe2x80x9cChemical-Mechanical Polishing in Semidirect Contact Mode,xe2x80x9d J Electrochem. Soc., Vol. 142, pp. 3845-3851.
Bramono, D. P. Y., and Racz, L. M., 1998, xe2x80x9cNumerical Flow-Visulization of Slurry in a Chemical Mechanical Planarization Process,xe2x80x9d Proc. 1998 CMP-MIC Conf., pp. 185-192.
Brown, N. J., Baker, P. C., and Maney, R. T., 1981, xe2x80x9cOptical Polishing of Metals,xe2x80x9d Proc. SPIE, Vol. 306, pp. 42-57.
Bulsara, V. H., Ahn, Y., Chandrasekar, S., Farris, T. N., 1998, xe2x80x9cMechanics of Polishing,xe2x80x9d ASME Journal of, Applied Mechanics, Vol. 65, pp. 410-416.
Chekina, O. G., Keer, L. M., and Liang, H., 1998, xe2x80x9cWear-Contact Problems and Modeling of Chemical Mechanical Polishing,xe2x80x9d J. Electrochem. Soc., Vol. 145, pp. 2100-2106.
Cook, L. M., 1990, xe2x80x9cChemical Processes in Glass Polishing,xe2x80x9d J. Non-Crystalline Solids, Vol. 120, pp. 152-171.
Cook, L. M., Wang, F., James, D. B., and Sethuraman, A. B., 1995, xe2x80x9cTheoretical and Practical Aspects of Dielectric and Metal Polishing,xe2x80x9d Semiconductor International, Vol. 18, pp. 141-144.
Komanduri, R., Umehara, N., and Raghanandan, M., 1996, xe2x80x9cOn the Possibility of Chemo-Mechanical Action in Magnetic Float Polishing of Silicon Nitride,xe2x80x9d ASME, Journal of Tribology, Vol. 118, pp. 721-727.
Kaufman, F. B., Thompson, D. B., Broadie, R. E., Jaso, M. A., Guthrie, W. L., Pearson, D. J., and Small, M. B., 1991, xe2x80x9cChemical-Mechanical Polishing for Fabricating Patterned W Metal Features as Chip Interconnects,xe2x80x9d J. Electrochem. Soc., Vol. 138, pp. 3460-3464.
Landis, H., Burke, P., Cote, W., Hill, W., Hoffman, C., Kaanta, C., Koburger, C., Lange, W., Leach, M., Luce, S., 1992, xe2x80x9cIntergration of Chemical-Mechanical Polishing into CMOS Integrated Circuit Manufacturing,xe2x80x9d Thin Solid Films, Vol. 220. pp. 1-7
Liu, C. -W., Dai, B. -T., Tseng, W. -T., and Yeh, C. -F., 1996, xe2x80x9cModeling of the Wear Mechanism during Chemical-Mechanical Polishing,xe2x80x9d J. Electrochem. Soc., Vol. 143, pp. 716-721.
Luo, Q., Ramarajan, S., and Babu, S. V., 1998, xe2x80x9cModification of Preston Equation for the Chemical-Mechanical Polishing of Copper,xe2x80x9d Thin Solid Films, Vol. 335, pp. 160-167.
Nakamura, T., Akamatsu, K., and Arakawa, N., 1985, xe2x80x9cA Bowl Feed and Double Sides Polishing for Silicon Wafer for VLSI,xe2x80x9d Bulletin Japan Soc. Precision Engg., Vol. 19, pp. 120-125.
Peters, L., 1998, xe2x80x9cPursuing the Perfect Low-k Dielectric,xe2x80x9d Semiconductor International, Vol. 21, pp. 64-74.
Runnels, S. R., 1994, xe2x80x9cFeature-Scale Fluid-Based Erosion Modeling for Chemical-Mechanical Polishing,xe2x80x9d J. Electrochem. Soc., Vol. 141, pp. 1900-1904.
Runnel, S. R., and Eyman, L. M., 1994, xe2x80x9cTribology Analysis of Chemical-Mechanical Polishing,xe2x80x9d J. Electrochem. Soc., Vol. 141, pp. 1698-1701.
Runnels, S. R., Kim, I., Schleuter, J., Karlsrud, C., and Desai, M., 1998, xe2x80x9cA Modeling Tool for Chemical-Mechanical Polishing Design and Evaluation,xe2x80x9d IEEE Tran. on Semiconductor Mfg., Vol. 11, pp. 501-510.
Stavreva, Z., Zeidler, D., Plotner, M., Drescher, K., 1995, xe2x80x9cChemical Mechanical Polishing of Copper for Multilevel Metallization,xe2x80x9d Appl. Surface Sci., Vol. 91, pp. 192-196.
Stavreva, Z., Zeidler, D., Plotner, M., Grasshoff, G., Drescher, K., 1997, xe2x80x9cChemical-Mechanical Polishing of Copper for Interconnect Formation,xe2x80x9d Microelectronic Engr., Vol. 33, pp. 249-257.
Steigerwald, J. M., Zirpoli, R., Murarka, S. P., Price, D., Gutmann, R. J., 1994, xe2x80x9cPattern Geometry Effects in the Chemical-Mechanical Polishing of laid Copper Structures,xe2x80x9d J Electrochem. Soc., Vol. 141, pp. 2842-2848.
Sundararajan, S., Thakurta, D. G., Schwendeman, D. W., Murarka, S. P., and Gill, W. N., 1999, xe2x80x9cTwo-Dimensional Wafer-Sacle Chemical Mechanical Planarization Models Based on Lubrication Theory and Mass Transport,xe2x80x9d J. Electrochem. Soc., Vol. 146, pp. 761-766.
Yu, T. -K., Yu, C. C., and Orlowski, M., 1993, xe2x80x9cA Statistical Polishing Pad Model for Chemical-Mechanical Polishing,xe2x80x9d Proc. 1993 IEEE Int. Electron Dev. Mfg., pp. 865-868.
Zhao, B., and Shi, F. G., 1999, xe2x80x9cChemical Mechanical Polishing in IC Process: New Fundamental Insights,xe2x80x9d Proc. 1999 CMP-MIC Conf, pp. 13-22.
Accordingly, it is an object of the present invention to provide a method of chemical mechanical polishing (CMP) that promotes increased material removal rate (MRR). More particularly, it is an object of the present invention to provide a method which operates in a contact mode at the interface between the CMP polishing pad and the wafer or substrate surface. Further, the present invention provides a method which identifies preferred CMP process parameters for increasing the MRR.
As will be described in detail below, the inventors have discovered that to increase the material removal rate, the CMP process must be operated in the contact mode at the interface between the wafer and the polishing pad. Hydroplaning at the interface is not a stable process mode in terms of the gimbaling point location, wafer curvature, and fluctuations in slurry flow. Accordingly, the important issue in CMP process design is to select process parameters to maintain the process in the stable contact regime. Further, the inventors have discovered that, within the contact mode, preferred process parameters may be identified according to a mathematical derivation as described below.
In general, a method of chemical mechanical polishing a surface of a wafer with a polishing pad is provided, comprising the steps of: rotating any one or both of the polishing pad and the wafer at a relative velocity vR; and urging the wafer and pad against each other at an applied pressure p, wherein the values of p and vR are such that the interface between the pad and the wafer are in the contact mode.
In another aspect of the present invention, a method of chemical mechanical polishing is provided where the following equation is satisfied:
vR/P≈C1/xcex7xe2x80x83xe2x80x83(1)
where vR is the relative velocity of the polishing pad and the wafer, p is the pressure applied to the wafer, and C1, is a constant that is related to the geometry of the polishing interface and machine design, and xcex7 is the viscosity of the slurry used in the particular CMP process, as described further below.
In a further aspect of the present invention, a method of chemical mechanical polishing is provided wherein the interfacial friction coefficient is monitored during the CMP process to maintain the interface between the wafer and the pad in the contact mode, and preferably to maintain the CMP process at the preferred operating parameters. For example, method of chemical mechanical polishing a surface of a wafer with a polishing pad is provided comprising the steps of: rotating any one or both of the polishing pad and the wafer at a relative velocity vR; urging the wafer and pad against each other at an applied pressure p; measuring the frictional forces generated by the pad and wafer during the polishing; determining the friction coefficient from said friction measurement; and controlling the values of p and vR to maintain the friction coefficient at a value of about 0.1 or greater during polishing.